The present invention relates to methods of manufacturing semiconductor devices having a gate insulating film made of a high dielectric constant material.
In recent years, advances have been made in reducing the size of MOSFETs (metal oxide semiconductor field effect transistors) that use silicon dioxide (SiO2: relative dielectric constant ε=3.9) or the like as the gate insulating film. As the size of the MOSFET is made progressively smaller, however, the capacitance (in other words, the gate insulating film capacitance) of the MOSFET drops, and as a result, the MOSFET no longer operates. Consequently, one solution that has been proposed for keeping the capacitance of the MOSFET from dropping is to reduce the film thickness of the gate insulating film of the MOSFET. However, if silicon dioxide is used as the gate insulating film, then reducing the thickness of the gate insulating film leads to the problem of increased gate leak current caused by tunneling current, and the MOSFET loses its ability to function as a transistor. As a result, it becomes difficult to further reduce the thickness of the gate insulating film. In other words, it is necessary that the film thickness of the gate insulating film of a MOSFET is kept above a certain level in order to prevent the capacitance of the MOSFET from dropping.
However, maintaining the film thickness of the gate insulating film above a certain level does not solve the problem of a drop in the capacitance of the MOSFET. Accordingly, another conceivable method for solving this problem is the use of a high-dielectric constant material (metal oxide) such as hafnium oxide (HfO2: relative dielectric constant ε=approximately 30) or zirconium oxide (ZrO2: relative dielectric constant ε=approximately 25) as the material for the gate insulating film. Using a metal oxide that has a high dielectric constant for the gate insulating film allows the physical film thickness to be increased while achieving a thin EOT (equivalent oxide (SiO2) thickness). To date, gate insulating films employing ZrO2, for example, have been disclosed (T. Yamaguchi et al., Study on Zr-Silicate Interfacial Layer of ZrO2-MIS Structure Fabricated by Pulsed Laser Ablation Deposition Method, Extended Abstracts of the 2000 International Conference on Solid State Devices and Materials, Sendai, Japan, August 2000, pg 228 to 229).
FIG. 11 is a cross-sectional view showing a conventional semiconductor device manufacturing method. More specifically, it shows is a method of forming a metal oxide film as the gate insulating film. Here, a device capable of performing sputtering is employed as the film deposition system. As shown in FIG. 11, a substrate 1 made of silicon is placed inside a chamber (not shown) of the film deposition system. Hafnium (Hf) metal is employed as the target 11. Also, a gas mixture 12 including argon (Ar) gas and oxygen (O2) gas is filled into the chamber. When a voltage is applied inside the chamber filled with the gas mixture 12, an electrical discharge occurs within the chamber, and this results in the formation of a metal oxide film 2 on the silicon substrate 1 due to reactive sputtering. If Hf metal is employed as the target 11, then the metal oxide film 2 that is formed directly on the silicon substrate 1 is theoretically a hafnium oxide film (HfO2). By controlling the sputtering time, an HfO2 thin film of about 3 to 10 nm thickness can be obtained as the metal oxide film 2.
However, in conventional methods for forming metal oxide films, the silicon substrate 1 and the metal oxide film 2, that is, the HfO2 film, are exposed to an atmosphere that includes oxygen gas (the gas mixture 12 in FIG. 11). Consequently, as shown in FIG. 12, in practice, silicon diffuses from the silicon substrate 1 into the HfO2 film and oxygen diffuses from the gas mixture into the HfO2 film and the silicon substrate 1. As a result, as illustrated in FIG. 13, an interface layer 3 is formed between the silicon substrate 1 and the metal oxide film 2 made of the HfO2 film. This interface layer 3 is conceivably a silicon-rich silicate (SixMyO2, where M is a metal (Hf in the case of FIG. 13) and x+y=1 (x>0, y>0)).
In other words, in conventional methods of forming metal oxide films, two layers, that is, a low-dielectric constant interface layer 3 and a high-dielectric constant HfO2 thin film (metal oxide film 2), are formed on the silicon substrate 1. This means that a capacitor with a low capacitance and a capacitor with a high capacitance are connected in series, and the overall capacitance of the transistor is dictated by the capacitance of the capacitor with the lower capacitance. Consequently, even if attempts are made to increase the capacitance by using a material that has a high dielectric constant, if an interface layer is formed on the substrate, then the capacitance of the entire transistor (that is, the entire gate insulating film including the interface layer) drops, and as a result, MOSFET performance cannot be increased.